The so-called (and marketing-branded) “blockchain technology” is promised to revolutionize every industry. Anything, they say, will become decentralized, free from middle men or government control. Services will thrive on various installments of the blockchain, and smart contracts will automatically enforce any logic that is related to the particular domain. I don’t mind having another technological leap (after the internet), and given that I’m technically familiar with the blockchain, I may even be part of it. But I’m not convinced it will happen, and I’m not convinced it’s going to be the next internet. If we strip the hype, the technology behind Bitcoin is indeed a technical masterpiece. It combines existing techniques (likes hash chains and merkle trees) with a very good proof-of-work based consensus algorithm. And it creates a digital currency, which ontop of being worth billions now, is simply cool. But will this technology will be mass-adopted, and will mass adoption allow it to retain the technological benefits it has? First, I’d like to nitpick a little bit – if anyone is speaking about “decentralized software” when referring to “the blockchain”, be suspicious. Bitcon and other peer-to-peer overlay networks are in fact “distributed” (see the pictures here). “Decentralized” means having multiple providers, but doesn’t mean each user will be full-featured nodes on the network. This nitpicking is actually part of another argument, but we’ll get to that. If blockchain-based applications want to reach mass adoption, they have to be user friendly. I know I’m being captain obvious here (and fortunately some of the people in the area have realized that), but with the current state of the technology,...

It is likely that you are developing some form of (web/RESTful) API, and in case it is publicly-facing (or even when it’s internal), you normally want to rate-limit it somehow. That is, to limit the number of requests performed over a period of time, in order to save resources and protect from abuse. This can probably be achieved on web-server/load balancer level with some clever configurations, but usually you want the rate limiter to be client-specific (i.e. each client of your API sohuld have a separate rate limit), and the way the client is identified varies. It’s probably still possible to do it on the load balancer, but I think it makes sense to have it on the application level. I’ll use spring-mvc for the example, but any web framework has a good way to plug an interceptor. So here’s an example of a spring-mvc interceptor: @Component public class RateLimitingInterceptor extends HandlerInterceptorAdapter { private static final Logger logger = LoggerFactory.getLogger(RateLimitingInterceptor.class); @Value("${rate.limit.enabled}") private boolean enabled; @Value("${rate.limit.hourly.limit}") private int hourlyLimit; private Map<String, Optional<SimpleRateLimiter>> limiters = new ConcurrentHashMap<>(); @Override public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { if (!enabled) { return true; } String clientId = request.getHeader("Client-Id"); // let non-API requests pass if (clientId == null) { return true; } SimpleRateLimiter rateLimiter = getRateLimiter(clientId); boolean allowRequest = limiter.tryAcquire(); if (!allowRequest) { response.setStatus(HttpStatus.TOO_MANY_REQUESTS.value()); } response.addHeader("X-RateLimit-Limit", String.valueOf(hourlyLimit)); return allowRequest; } private SimpleRateLimiter getRateLimiter(String clientId) { if (limiters.containsKey(clientId)) { return limiters.get(clientId); } else { synchronized(clientId.intern()) { // double-checked locking to avoid multiple-reinitializations if (limiters.containsKey(clientId)) { return limiters.get(clientId); } SimpleRateLimiter rateLimiter = createRateLimiter(clientId); limiters.put(clientId, rateLimiter); return rateLimiter; } } } @PreDestroy public void...

Spring Boot Admin is a cool dashboard for monitoring your spring boot applications. However, setting it up is not that trivial. The documentation outlines two options: Including a client library in your boot application that connects to the admin application – this requires having the admin application deployed somewhere public or at least reachable from your application, and also making your application aware that it is being monitored. Using cloud discovery, which means your application is part of a service discovery infrastructure, e.g. using microservices Both are not very good options for simpler scenarios like a monolithic application being run on some IaaS and having your admin application deployed either on a local machine or in some local company infrastructure. Cloud discovery is an overkill if you don’t already need it, and including a client library introduces the complexity of making the admin server reachable by your application, rather than vice-versa. And besides, this two-way dependency sounds wrong. Fortunately, there is an undocumented, but implemented SimpleDiscoveryClient that let’s you simply run the Spring Boot Admin with some configuration on whatever machine and connect it to your spring boot application. The first requirement is to have the spring boot actuator setup in your boot application. The Actuator exposes all the needed endpoints for the admin application to work. It sounds trivial to setup – you just add a bunch of dependencies and possibly specify some config parameters and that’s it. In fact, in a real application it’s not that easy – in particular regarding the basic authentication for the actuator endpoints. You need a separate spring-security (in addition to your...

“What are some areas you are particularly interested in” – recruiters (head-hunters) tend to ask that question a lot. I don’t have a good answer for that – I’ll know it when I see it. But I have a list of areas that I wouldn’t like to work in. And one of them is gambling. Several years ago I got a very lucrative offer for a gambling company, both well paid and technically challenging. But I rejected it. Because I didn’t want to contribute to abusing peoples’ weaknesses for the sake of getting their money. And no, I’m not a raging Marxist, but gambling is bad. You may argue that it’s a necessary vice and people need it to suppress other internal struggles, but I’m not buying that as a motivator. I felt it’s unethical to write code that does that. Like I feel it’s unethical to profile users’ behaviours and “read” their emails in order to target ads, or to write bots to disseminate fake news. A few months ago I was part of the campaign HQ for a party in a parliamentary election. Cambridge Analytica had already become popular after “delivering Brexit and Trump’s victory”, that using voters’ data in order to target messages at them sounded like the new cool thing. As head of IT & data, I rejected this approach. Because it would be unethical to bait unsuspecting users to take dumb tests in order to provide us with facebook tokens. Yes, we didn’t have any money to hire Cambridge Analytica-like companies, but even if we had, is “outsourcing” the dubious practice changing anything? If...

Sometimes we need to let users sign something electronically. Often people understand that as placing your handwritten signature on the screen somehow. Depending on the jurisdiction, that may be fine, or it may not be sufficient to just store the image. In Europe, for example, there’s the Regulation 910/2014 which defines what electronic signature are. As it can be expected from a legal text, the definition is rather vague: ‘electronic signature’ means data in electronic form which is attached to or logically associated with other data in electronic form and which is used by the signatory to sign; Yes, read it a few more times, say “wat” a few more times, and let’s discuss what that means. And it can bean basically anything. It is technically acceptable to just attach an image of the drawn signature (e.g. using an html canvas) to the data and that may still count. But when we get to the more specific types of electronic signature – the advanced and qualified electronic signatures, things get a little better: An advanced electronic signature shall meet the following requirements: (a) it is uniquely linked to the signatory; (b) it is capable of identifying the signatory; (c) it is created using electronic signature creation data that the signatory can, with a high level of confidence, use under his sole control; and (d) it is linked to the data signed therewith in such a way that any subsequent change in the data is detectable. That looks like a proper “digital signature” in the technical sense – e.g. using a private key to sign and a public key to...

What happens when a senior developer becomes…more senior? It often happens that they get promoted to “architect”. Sometimes an architect doesn’t have to have been a developer, if they see “the bigger picture”. In the end, there’s often a person that holds the position of “architect”; a person who makes decisions about the architecture of the system or systems being developed. In bigger companies there are “architect councils”, where the designated architects of each team gather and decide wise things… But I think it’s a bad idea to have a position of “architect”. Architect is a position in construction – and it makes sense there, as you can’t change and tweak the architecture mid-project. But software architecture is flexible and should not be defined strictly upfront. And development and architecture are so intertwined, it doesn’t make much sense to have someone who “says what’s to be done” and others who “do it”. It creates all sorts of problems, mainly coming from the fact that the architect doesn’t fully imagine how the implementation will play out. If the architect hasn’t written code for a long time, they tend to disregard “implementation details” and go for just the abstraction. However, abstractions leak all the time, and it’s rarely a workable solution to just think of the abstraction without the particular implementation. That’s my first claim – you cannot be a good architect without knowing exactly how to write the whole code underneath. And no, too often it’s not “simple coding”. And if you have been an architect for years, and so you haven’t written code in years, you are almost certainly...